Electronic communication devices such as portable cellular telephones usually comprise an antenna arrangement to transmit and receive electromagnetic waves. In recent years, the number of antenna elements within antenna arrangements has increased to enable communication devices to communicate over a greater number of radio frequency bands.
An antenna arrangement may include at least one antenna element mounted on a ground plane (typically the printed wiring board of the communication device). Due to electromagnetic coupling between the antenna element and the ground plane, the height of the antenna element above the ground plane affects the bandwidth of the antenna element. Specifically, the bandwidth of the antenna element decreases as the height of the antenna element above the ground plane decreases. Consequently, the height of the antenna element above the ground plane must usually be greater than a minimum threshold height to ensure reasonable bandwidth. For example, in a mobile telephone an internal antenna such as a PIFA or loop antenna will usually have a minimum threshold height which is usually greater than 4 mm dependent upon the bandwidth to be covered. Consequently, electromagnetic coupling between an antenna element and a ground plane is one factor which determines the volume of space required for an antenna arrangement within an electronic communication device.
If there is more than one antenna element mounted on the ground plane, electromagnetic coupling may occur between the antenna elements. This electromagnetic coupling may affect the impedance (and hence the resonant frequency) of the antenna elements. This problem may be particularly acute in communication devices which include one or more moveable antenna elements. One current solution to this problem is to physically separate the antenna elements as much as possible. However, one disadvantage associated with this solution is that it may increase the size of the antenna arrangement. Another solution to this problem is to provide additional electronic circuitry to minimise the effects of electromagnetic coupling. One example of additional electronic circuitry is an isolator which may be used to minimise the effects of antenna impedance changes presented to the connected communications circuitry. An isolator is usually positioned between the power amplifier and the antenna element to prevent unwanted signals from affecting the transmitter output. However, one disadvantage associated with additional electronic circuitry is that it may increase losses which result in increased power consumption. Another disadvantage associated with additional electronic circuitry is that it may increase the cost of the electronic communication device.
Consequently, it is desirable to provide an alternative antenna arrangement.